Which cacti was the most fit in this specific environment? How do you know?

The cactus on the right, because it survives the whole time.

Explanation:

the spiked cacti.

Explanation:

the cacti without spikes was eaten by the deer therfore withering away where as the one with spikes survied.

Option (4)

Explanation:

Index fossils are defined as those fossils that existed for a shorter period of time and are widely distributed on earth. They are abundant. These index fossils because of their short vertical range, are helpful in the determination of the age of the rocks and also to identify the major events that have taken place in the past. For example, the ammonites.

In the given data, the layers comprising of fossils namely A, B, C, D, E and F are provided, where the youngest layer is layer 1 and the oldest layer is layer 6.

From this set of fossils, we can mark that the index fossil within these rock formations is the fossil E. It is because E is the only fossil that is found in the layer 2 which is abundant and is found in a rock layer that belongs to a particular age.

All other fossils i.e. fossils A, B, C, and D  are found in two of the layers, which does not satisfy the condition of an index fossil. They are found in different rock layers belonging to a different time.

Hence, the correct answer is option (4) i.e. fossil E.